The present invention relates to a loudspeaker for a portable communication instrument such as a portable telephone.
The loudspeaker provided in the movable telephone is used as a speaker for reproducing a calling information sound in response to a calling signal and as a receiver for converting a voice signal into voice.
In order to reproduce a calling information sound such as music sound in response to the calling signal, a loudspeaker having a wide frequency range of 20 Hzxcx9c20 KHz is mounted in the portable telephone. On the other hand, another loudspeaker having a narrow frequency range of 300 Hzxcx9c3 KHz for converting the voice signal into the voice is also provided in the portable telephone.
To miniaturize the portable telephone, two loudspeakers having wide and narrow frequency ranges are provided in one case.
FIG. 2 shows such a composite speaker. The speaker has a cylindrical case 1 having an annular projection 1b inwardly projected from the inside wall of an upper portion of the case. An annular yoke 2 is secured to the inside wall of the projection 1b at a cylindrical portion 2a. An annular permanent magnet 3 is secured to the underside of the yoke 2. A pole piece 4 is secured to the underside of the permanent magnet 3 at a flange 4a. 
A first diaphragm 6 is secured to the upper surface of the yoke 2 at a peripheral portion thereof by a first protector 9. A second diaphragm 8 is secured to the underside of the case 1 by a second protector 10. On the underside of the first diaphragm 6, a first voice coil 5 is secured, and a second voice coil 7 is secured to the upper surface of the second diaphragm 8. The first voice coil 5 is disposed in a magnetic gap 11 between the inside wall of the yoke 2 and the outside wall of a center pole 4b of the pole piece 4. The second voice coil 7 is disposed in a magnetic gap 12 between the inside wall of the cylindrical portion 2a of the yoke 2 and the flange 4a of the pole piece 4. A plurality of sound discharge holes 13 and 14 are provided in the first protector 9 and in the second protector 10, and a plurality of back holes 1a are formed in the case 1.
The first voice coil 5 and the first diaphragm 6 are used for a receiver for converting a voice signal into voice, and the second voice coil 7 and the second diaphragm 8 are used for a speaker for reproducing a calling information sound in response to a calling signal. When a calling signal is applied to the portable telephone, a calling information sound such as music sound having a wide frequency range is reproduced as a calling sound by the second diaphragm. A voice signal is converted into voice by the first diaphragm.
The voice coil driving force F is expressed as follows.
F=BLi
where B is the density of magnetic flux, L is the effective length of coil, i is the coil current.
In the loudspeaker, the magnetic flux is used in two magnetic gaps 11 and 12. Consequently, the density of the magnetic flux reduces compared with the case where the magnetic flux is applied to a single magnetic gap. Therefore, the coil driving force F becomes small, causing the sound pressure to reduce.
An object of the present invention is to provide a loudspeaker which may produce sufficient volume without increasing the size of the loudspeaker.
According to the present invention, there is provided a loudspeaker comprising a case, a first diaphragm provided in the case, a second diaphragm provided in the case, a first voice coil for vibrating the first diaphragm, a second voice coil for vibrating the second diaphragm, a first permanent magnet for applying a magnetic flux to the first voice coil, and a second permanent magnet for applying a magnetic flux to the second voice coil.
Each of the first and second voice coils has an annular shape.
The loudspeaker further comprises an annular first magnetic gap, and an annular second magnetic gap, each of the magnetic gaps is formed between opposite magnetic members which are engaged with either of the first and second permanent magnets, and each of the first and second voice coils is disposed in either of first and second magnetic gaps.
The first permanent magnet has an annular shape, and the second permanent magnet has a disc shape, the first permanent magnet is disposed outside the first and second magnetic gaps, and the second permanent magnet is disposed inside the magnetic gaps.
These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.